1. Field of the Invention
This invention relates generally to the field of space heating, and more specifically to an improved high-efficiency furnace for use in heating mobile homes.
2. Description of the Related Art
To heat manufactured housing, such as mobile homes, forced hot air furnaces that are small and compact have been developed. These furnaces are often located in a closet or alcove of the mobile home. A blower or fan is typically used to draw room air to be heated into the furnace. The air is then blown downwardly past an enclosed heat exchanger, causing it to be heated. Heated air is then forced out of the furnace at its base, which is coupled to an arrangement of hot air ducts. These ducts typically run underneath the floor of the mobile home and include a number of outlets. Heated air is thus delivered to various parts or rooms of the mobile home.
Partially disposed within the heat exchanger, which also acts as a combustion chamber, is a burner assembly that typically runs on fuel gas (natural gas or propane) and includes a gas control and an ignitor. The burner assembly introduces a fuel-air mixture inside the combustion chamber which is ignited and burned, thereby heating the combustion chamber. Combustion gases and other reactants generated during the combustion process flow upwardly through the combustion chamber, exiting the furnace through an upwardly extending vent or chimney. As the hot combustion gases are moving upwardly through the heat exchanger, room air is being forced downwardly past the heat exchanger by the fan, as described above.
As shown in commonly owned U.S. Pat. No. 4,924,848, other heat exchangers in addition to the combustion chamber may also be utilized to improve the efficiency of the furnace. For example, a radiator which receives combustion gases from the combustion chamber may be disposed adjacent to the fan in order to extract additional heat from the combustion gases before they are discharged from the furnace. More specifically, room air is first drawn past the radiator before it enters the fan, so the air is pre-heated. This pre-heated air is then blown downwardly by the fan past the combustion chamber where it is further heated before being discharged through the furnace outlet.
An intermediary heat exchanger, which interconnects the combustion chamber and the radiator, may also be provided. The intermediary heat exchanger may consist of a plurality of horizontal tubes that extend from the top of the combustion chamber to the base of the radiator, allowing combustion gases to flow therebetween. The tubes, moreover, may be disposed directly below the downward facing fan outlet so that the air being heated first flows past the intermediary heat exchanger and then past the combustion chamber.
Although the furnace disclosed in U.S. Pat. No. 4,924,848 represents a significant improvement in compact furnace designs, it nonetheless has several limitations. First, while the radiator indeed provides some improvement to the operating efficiency of the furnace, its design nonetheless has several drawbacks. In particular, the thin, box-like radiator is typically mounted along one of the side walls of the furnace housing, facing one of the fan inlets. That is, the radiator is positioned parallel to the direction of air entering the furnace, but orthogonal to the direction of the air entering the fan. Due to the relative positioning of the fan and the radiator, air being drawn into the fan typically flows across only one surface of the radiator, limiting the amount of preheating performed by the radiator. In addition, a significant amount of room air enters the fan through its second inlet which is opposite the radiator and is thus not preheated at all.
Furthermore, mobile homes obviously come in many different sizes and floor plans. Accordingly, it is desirable to provide a furnace that may be easily modified so as to increase or decrease its thermal output depending on its intended environment. Nevertheless, the furnace disclosed in U.S. Pat. No. 4,924,848 does not lend itself to easy modification. For example, all three heat exchanger components must be installed and used for the furnace to operate, making it extremely difficult, if at all possible, to adjust the furnace's thermal output. It is also desirable to provide a single furnace design that may accommodate either home heating oil or natural gas burners with little, if any, modification. The configuration of the burner assembly, however, varies significantly depending on whether it burns oil or gas and also whether it operates under pressure or natural draft conditions. Due to these configuration differences, the inlet formed in the combustion chamber must be sized for the particular burner assembly being installed. For example, if the combustion chamber is to accept a natural draft burner which typically utilizes a venturi and pilot assembly, the inlet configuration includes a relatively small diameter opening. This inlet configuration, however, prevents the combustion chamber from accepting, for example, an oil-fired burner which typically includes an outer tube and mounting flange, thereby requiring a different (often larger) inlet configuration. Thus, a different chamber design is required for each burner type.
The burner assembly itself may also introduce several inefficiencies to the operation of the furnace. For example, as mentioned above, in order to permit the burner assembly to extend partially into the combustion chamber, a large inlet is often formed in the base of the combustion chamber. Ideally, the burner assembly generates a steady, round ball of flame, centrally disposed within the drum so as to heat the drum evenly. Conventional burner assemblies, which often include flat, rectangular spreader plates, however, typically produce fluctuating and/or misshaped flames, resulting in uneven heating of the combustion chamber. This uneven heating of the combustion chamber, in turn, results in less efficient heat transfer to the room air being blown downwardly by the fan.
Additionally, the lower airbox which is mounted to the base of the combustion chamber and delivers combustion air to the burner assembly often introduces undesirable flow characteristics, such as nonuniformity, into the airflow being provided to the burner assembly. This nonuniformity may result in inefficient and unstable operation of the burner assembly. For example, the airflow may fluctuate over time, causing instabilities in the operation of the burner assembly. These instabilities often reduce the operating efficiency of the furnace.
With many furnaces a blower is provided to increase the static pressure of the air being supplied to the burner assembly, thereby improving the heat transfer process by establishing forced convection. The blower additionally forces the combustion gases and reactants through the furnace and out the vent. To generate the requisite increase in static pressure economically, centrifugal sheet metal blowers are typically employed. These types of blowers, however, present several disadvantages. For example, a centrifugal blower, by design, tends to have a small outlet with a non-uniform, high velocity airflow, which often disrupts the flow of air into the burner (a highly undesirable effect for the reasons described above). The placement and installation of a centrifugal blower also results in added complexity to the furnace. Centrifugal blowers are also relatively expensive components, increasing the overall costs of the furnace.
Oil-fired furnaces often include a ceramic combustion chamber to protect the heat exchanger from the intense, often localized flame of an oil-fired burner. These chambers, which are typically disposed in an upright manner within vertical heat exchangers, include a side opening form receiving the oil-fired burner and an opening in the top end for releasing the flame and heat. Although these designs are generally acceptable, the applicants herein have identified several disadvantages. For example, the flame and heat are mainly directed upwardly causing non-uniform heating of the heat exchanger. Additionally, these chambers are problematic to install and position within the heat exchanger due, at least in part, to the large opening required for their assembly within the heat exchanger.